1. Field of the Invention
This invention relates to a novel positive photoresist composition and to the method for providing positive photoresist layers. In particular the invention deals with alkali soluble resin-diazo ketone photoresists that have good wetting properties on various substrates. The invention is particularly useful in that the photoresist compositions provided obviate the adverse effects that are characterized as striations which occur and result in a nonuniform film and consequently in a relatively inferior semiconductor device.
2. Description of the Prior Art
Photoresists are materials which change their solubility in response to a developer solution after the photoresist has been exposed, such as to ultraviolet radiation. Photoresist compositions comprise a photosensitive compound (hereafter sometimes called sensitizer or photosensitizer), a film forming polymeric resin and a solvent. As a consequence of the exposure to radiation of the photoresist, a different solubility rate results between the exposed and unexposed (masked over) portions of a resist film that yields a surface relief pattern after the development. Those photoresists which become more soluble in the exposed regions are referred to as positive photoresists.
Positive photoresists are typically comprised of an aqueous alkaline soluble resin, such as novolac resin or poly p-hydroxy styrene, and a diazonaphthoquinone sulfonic acid ester sensitizer. The resin and sensitizer are applied such as by spin coating from an organic solvent or solvent mixture onto a substrate, such as silicon wafers and chrome plated glass plates. The developer used to process the positive photoresists are aqueous alkaline solutions, such as sodium silicate, potassium hydroxide, tetramethyl ammonium hydroxide and ammonium hydroxide. The developer removes the areas of the coated photoresist film that has been exposed to light or other form of irradiaton so as to produce a pattern in the photoresist film.
The application of a photosensitive film to various substrates is an essential step in the fabrication of integrated circuits. The substrates are generally silicon wafers which may have a thin oxide coating or other coating such as a silicon nitride or aluminum. The photosensitive film is used to pattern the substrate in a series of steps including exposure, development and substrate etch. It is essential that the mask pattern be accurately reproduced in the substrate etch pattern. To achieve this high degree of accuracy, it is essential that the photoresist film be of uniform thickness. Unfortunately, many photoresist formulations do not provide uniform coating, either because they do not wet and uniformly spread out over the substrate or because of striations in the film that are caused by the mechanics of spin casting. Striations are radial variations in film thickness that can be as much as 200 to 600 .ANG..
A key component which determines the wetting, spreading and striations performance of a photoresist is the solvent.
Photoresists are applied to silicon wafers having a variety of surfaces comprising silicon, polycrystalline silicon, silicon dioxide, silicon nitride, aluminum, etc. Many of these surfaces are treated with adhesion promoters which help to prevent undercut of the photoresist film during wet etching of the exposed, developed wafer. This can cause the wafer pattern to deviate from the photomask geometries. A typical adhesion promoter widely used in the industry is hexamethyldisilazane. Although this adhesion promoter limits undercut during wet etch, it makes the surface more difficult to wet. The extent of this dewetting effect is dependent upon the mode and time of contact of the silicon wafer with the hexamethyldisilazane. Wafers which have been treated with hexamethyldisilazane vapor for periods of greater than ten minutes can be particularly difficult to wet.
The extent of striations is highly dependent upon the nature of the solvent. The factors which affect striations are not fully understood. There is evidence that the vapor pressure of the solvent, the degree of interaction of solvent with the resin, and the acceleration of the wafer during application have some effect on the formation of striations.
It has been observed that some solvents are more effective than others in providing improved wetting of the substrate. For example, n-butyl acetate has excellent wetting properties but gives very bad striations. Diglyme gives no striations but has extremely poor wetting characteristics. Xylene has been used to limit striations but it has adverse effects on wetting.
In addition, all these materials are poor solvents for such typical sensitizers as 2,3,4-tris(6-diazo-5,6-dihydro-5-oxo-1-naphthalenesulfonyloxy)benzophenone or 2-hydroxy-3,4-bis(6-diazo-5,6-dihydro-5-oxo-1-naphthalenesulfonyloxy)benzo phenone or 2,4-bis(6-diazo-5,6-dihydro-5-oxo-1-naphthalenesulfonyloxy)benzophenone, which are the preferred sensitizers for a high sensitivity positive photoresist.
The solvent most commonly used in the formulation of commercial positive photoresists is cellosolve acetate. Evidence has been disclosed that this solvent has significant toxic effects. In addition, resist formulations made using this solvent show striations. Some commercial photoresists contain a mixture of cellosolve acetate, xylene and n-butyl acetate. The xylene and n-butyl acetate may be used to alter the wetting and striations performance of the product. Although the n-butyl acetate improves wetting, the xylene does not totally eliminate striations. Surface active agents added to certain formulations improve wetting and decrease striations. A serious limitation of surface active agents is that they are not volatile and do not evaporate with the solvent during the spin casting step or in the subsequent oven bake between 90.degree.-110.degree. C. generally used to remove the solvent. The presence of residual surface active agents even at very low levels of less than 0.1% adversely affects the performance of the resist. This residual surfactant seriously affects resolution and may lead to the loss of the pattern where dimensions are less than two microns.
In addition to providing no striations and good wetting and spreading properties, the solvent must be a good solvent for both the resin and sensitizer. The preferred photosensitive compounds having more than one diazonaphthoquinone moiety are generally of low solubility. Suitable solvents which possess all of the desired properties, including solubility, wetting, and low striations, are difficult to find. In order to obtain the requisite sensitivity and contrast, it is necessary to have a sufficiently high concentration of photosensitive compound in the resist formulation to prevent dissolution of the unexposed resist during development. To overcome this difficulty, it is a common practice to use a mixture of sensitizers consisting of less soluble and more soluble types or even to operate with the solution supersaturated relative to the less soluble but more effective sensitizer component. This can lead to short shelf life and precipitation of the sensitizer prior to or during use of the product.
It is accordingly apparent that a need exists for a photoresist formulation which provides low striations and excellent wetting properties when spun cast on various substrates used in integrated circuit manufacture. The key element in this invention is the solvent composition which consists of cyclopentanone, or cyclopentanone and cyclohexanone with an aliphatic alcohol. These solvent components when used in certain critical ratios provide unexpectedly good wetting of various substrates and low striation films when spun applied.